This invention relates to an air/fuel ratio control apparatus in an internal combustion engine which is particularly suitable for correcting a deviation from a desired optimum air/fuel ratio occurring during transitional periods of engine operation.
FIG. 4 schematically illustrates a typical example of a known air/fuel ratio control apparatus in an internal combustion engine. In this figure, an internal combustion engine illustrated includes a plurality of engine cylinders 1 (though only one is illustrated) each defining therein a combustion chamber 1a to which an intake manifold 2 is connected. A throttle valve 3 is disposed in the intake manifold 2 for controlling the flow rate of an intake air supplied from the ambient atmosphere to the combustion chamber 1a through the intake manifold 2. An engine operating parameter sensing means 4 in the form of a pressure sensor is mounted on the intake manifold 2 at a location downstream of the throttle valve 3 for sensing the internal pressure or intake pressure in a portion of the intake manifold 2 downstream of the throttle valve 3 and generating a corresponding output signal to an engine control unit (ECU) 9 which controls various aspects of engine operation as referred to in detail later. A temperature sensor 10 is mounted on the intake manifold 2 for sensing the temperature of the engine or the temperature of an engine coolant and generating a corresponding output signal to the ECU 9. A rotational speed sensor 5 senses the rotational speed or the number of revolutions per minute of the engine and generates a corresponding output signal in the form of pulses to the ECU 9. A fuel injector 6 is provided in the intake manifold 2 at a location upstream of the throttle valve 3 for injecting an amount of fuel into the intake pipe 2. A throttle sensor 8 is operatively connected with the throttle valve 3 for sensing the opening degree of the throttle valve 3 and generating a corresponding output signal to the ECU 9.
The ECU 9 includes an analog-to-digital (A/D) converter 91 to which the output signals from the pressure sensor 4, the throttle sensor 8 and the temperature sensor 10 are input, an interface circuit 92 which receives the pulsated output signal from the rotational speed sensor 5 and converts the level thereof into an appropriate level, a microprocessor 93, a ROM 94 for storing programs including basic data and the like to be executed by the microprocessor 93 for controlling engine operation, a RAM 95 for temporarily storing data, information and the like calculated or processed by the microprocessor 93, and an output circuit 96 for driving the fuel injector 6. The microprocessor 93 executes a control program stored in the ROM 94, calculates an appropriate amount of fuel required to be supplied to the intake manifold 2 on the basis of data and information input thereto from the various sensors 4, 5, 8 and 10 via the A/D converter 91 and the interface circuit 92, and controls the fuel injector 6 through the output circuit 96 so that the injector 6 injects the amount of fuel thus calculated into the intake manifold 2. Specifically, the microprocessor 93 properly controls the width of a drive pulse generated by the output circuit 96 for driving the fuel injector 6 so as provide the amount of fuel calculated.
The operation of the known apparatus will now be described in detail with reference to a flow chart illustrated in FIG. 5. First in Step 401, the pulsated output signal of the rotational speed sensor 5 representative of the number of revolutions per minute of the engine Ne is read into the microprocessor 93 through the interface circuit 92. In Step 402, the output signal of the pressure sensor 4 representative of the intake pressure (absolute pressure) in the intake manifold 2 is read into the microprocessor 93 through the A/D converter 91. In Step 403, based on the information read into the microprocessor 93 in Steps 401 and 402, the microprocessor 93 calculates a basic amount of fuel Q.sub.0 to be injected into the intake pipe 2 using the following equation: EQU Q.sub.0 =K.sub.1 .times.Pb.times..eta.v
where K.sub.1 is a constant; and .eta.v is a charging efficiency which is predetermined on the basis of the intake pressure Pb in the intake manifold 2 and the rotational speed (rpm) of the engine.
Subsequently in Step 404, the microprocessor 93 converts the amount of fuel Q.sub.0 thus calculated into a corresponding pulse width .tau. for driving the fuel injector 6 according to the following equation: EQU .tau.=K.sub.2 .times.Q.sub.0
where K.sub.2 is a constant.
With the above-described known air/fuel ratio control apparatus, however, the fuel supply from the fuel injector 6 into the intake manifold 2 is cut off during engine deceleration from the standpoints of fuel economy, emission control and the like, as taught by Japanese Patent Publication No. 55-4217. In this case, however, when the engine temperature as sensed by the temperature sensor 10 is low, for example below the freezing point, the cut-off of the fuel supply is cancelled or stopped for the purpose of ensuring stable engine operation. This results in the following drawbacks:
When the driver of a vehicle releases the acceleration pedal and depresses the brake pedal to stop the running vehicle, the throttle valve 3 is rapidly closed so that the intake pressure in the intake manifold 2 downstream of the throttle valve 3 is suddenly decreased to a significant extent, thus impairing combustion of the air/fuel mixture in the combustion chamber 1a of the cylinder 1. As the vehicle speed and the rpm of the engine decrease, the absolute value of the intake pressure in the intake pipe 2 downstream of the throttle valve 3 gradually increases after the sudden fall thereof. Accordingly, during the period immediately after the rapid closure of the throttle valve 3, such a sudden fall in the intake pressure cannot be sensed quickly or instantaneously but with a certain time delay, so the amount of intake air actually sucked into the cylinder 1 becomes greater than that which is calculated based on the intake pressure sensed by the pressure sensor 4. As a result, the actual air/fuel ratio of the mixture becomes much leaner than the optimum air/fuel ratio for the actual intake air sucked into the cylinder 1 through the intake manifold 2. In addition, the decreasing rpm of the engine gradually increases the intake pressure after the closure of the throttle valve 3, thereby reducing the rate of evaporation of fuel in the form of gasoline in the intake manifold 2 or in the combustion chamber 1a. This also serves to further decrease the air/fuel ratio of the mixture. As a result, the operation of the engine, which is operating in particular at low speed such as when it is idling, is greatly impaired. In this case, the change in the intake pressure inside the intake manifold 2 upon engine deceleration is substantially slow as compared with that at the time of engine acceleration, so a conventional method of correcting the air/fuel ratio based on a change in the intake pressure during transitional operating periods of the engine such as accelerating periods is not feasible for this situation.